1. Field of the Invention
The present invention relates to a polysilicon film and fabrication methods thereof, and in particular relates to directly fabricating a polysilicon film at a low temperature and a plasma chemical vapor deposition apparatus utilized.
2. Description of the Related Art
Currently, Solid Phase Crystallization or Excimer Laser Annealing (ELA) methods are utilized to form a polysilicon thin-film, such that the amorphous silicon on a thin film is crystallized into polysilicon by high temperature annealing. However, both processes have disadvantage.
For Solid Phase Crystallization, a silicon wafer or Quartz (SiO3) must be utilized as a substrate due to requirement for a high crystallization temperature. Because the required materials are relatively expensive, they are not suitable for mass production.
As for Excimer Laser Annealing, although crystallization temperature may be reduced, the cost of required apparatuses is relatively high. In addition, time required for the procedure is less to be desired.
Additionally, in recent years, Plasma Enhanced Chemical Vapor Deposition (PE-CVD) and Hot Wire Chemical Vapor Deposition (HW-CVD) have been developed as direct deposition methods for forming polysilicon thin film. Nevertheless, during the preliminary stage of polysilicon thin film deposition, the nucleation density is relatively low, requiring several thousands Armstrong (>1000 Å) to be deposited to form polysilicon thin film of desired crystallization. Specifically, disorder arrangement of silicon atoms takes place at the interface.
In addition to the direct deposition methods, Metal-Induced Lateral Crystallization (MILC) has been developed to deposit a relatively thinner layer of polysilicon at a slower speed, to be used as a seed layer for subsequent deposition of amorphous silicon. The speed of gas flow utilized in depositing the polysilicon is slower than that normally used in depositing the amorphous silicon by several folds. After the polysilicon has been deposited, amorphous silicon is deposited on the polysilicon to an appropriate thickness and is annealed in a furnace at 600° C., so that the amorphous silicon is crystallized into polysilicon. Since the seed layer already exists, the amorphous silicon can be transformed into polysilicon in a relatively short period of time. However, since it takes a long time to form the seed layer at a slower speed, there is relatively little time savings for the anneal process. Furthermore, the method does not suit mass production due to process challenges involved given the overly high co-melting point of metal and silicon and problems of contamination of the thin film by the metal. In addition, outcome of an overly high temperature of the substrate due to seed layer application may lead to additional processing problems and challenges.